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Studying mammals: The social climbers

2 Who are the anthropoids?

As you know, the suborder Anthropoidea includes monkeys, apes and humans. You will be aware from reading LoM that taxonomists group monkeys according to the shape of their nose: Old World monkeys are catarrhines and New World monkeys are platyrrhines [p. 264]. In fact, apes and humans share many traits with Old World monkeys, so they too belong to the catarrhrines, whereas the New World monkeys are sufficiently distinct to be contained within a grouping of their own. DA also mentions other anatomical differences between catarrhines and platyrrhines, notably their dental formulae [p. 266].

Old World monkeys, such as baboons, mandrills, mangabeys, guenons, macaques, colobus and langurs, are found in Africa and Asia; they belong to a single family (Cercopithecidae). The marmosets, tamarins and capuchin-like monkeys of the New World are found in Central and South America; they comprise two related families. Apes (gibbons, orangutans, gorillas, chimpanzees and bonobos) and humans are the subject of course S182_10, so I will say little about them here. As DA points out in LoM p. 272, apart from humans, very few primates are found 'outside the tropics', i.e. the zone around the Equator between the Tropics of Cancer and Capricorn. The regions between the hot tropical zone and the cold polar regions have moderate temperatures and comprise the temperate zones.

Question 1

From your reading of LoM, identify monkey species that are found in the temperate zones and state whether they are Old or New World monkeys.

Answer

You may have cited Japanese macaques [p. 272], rhesus monkeys [p. 272], Barbary macaques [p. 274], and golden, Yunnan and Guizhou snub-nosed monkeys [pp. 266-267]. They are all species of Old World monkeys. (The Barbary macaques include the famous 'apes' of Gibraltar.)

Anthropoids are long-living animals that typically produce single offspring and have a lengthy juvenile period before they reach sexual maturity. Most anthropoid primates are omnivorous, but as they diversified to occupy different ecological niches, some species became more specialised. Some eat mainly fruit (the guenons and spider monkeys), or seeds (the sakis), or leaves (the colobus, langurs and howler monkeys), or even grasses (the geladas), with associated adaptations to their digestive systems and dentition. You will recall from the TV programme that bearded sakis can crack open seeds with their teeth, an ability that is described in more detail in LoM pp. 252-253. The degrees of specialisation, however, are minor compared with those found in other mammals, such as the panda, as all species are able to revert to other foods when their preferred food items are unavailable.

Question 2

From your reading of LoM, recall another plant item that is an important food for some monkeys and name the species that feed on it.

Answer

Exudates from trees that are referred to as gum. Gum is a sticky, rubbery substance, formed when sap oozing from a tree solidifies in air. Gum is a major food source for marmosets. It's also important in bush-babies.

Activity 1

Watch the TV programme from 10.48-14.58 and read LoM p. 258. Observe how marmosets use their claws to grip onto the tree-trunks and their teeth to gnaw the bark. Write a description, in about 200 words, of how marmosets use their claws and teeth to obtain food and explain why their methods of obtaining food differ from those of other monkeys.

Answer

Marmosets are tiny, enabling them to venture onto thin twigs that cannot support the weight of larger monkeys. Their light body enables them to walk along twigs without shaking and alerting their prey. A marmoset approaches its prey - an insect - stealthily, placing each foot slowly and carefully. Then it pounces, using its stereoscopic vision to judge the striking distance with great accuracy.

Monkeys use their hands and feet, and in some species their tails, to grasp branches as they climb in the trees. The hands and feet of marmosets are big enough only to grasp twigs, and rather than nails they have claws that dig into the bark of larger branches and tree trunks, giving a firm hold.

Although marmosets feed on insects, as do many other primates, their main diet is unusual. Marmosets feed on gum; their caecum is enlarged to aid digestion and they have chisel-like incisors that can gnaw holes in tree bark, causing sap to ooze out. The wounds heal, sealing in the sap, so every day marmosets need to gnaw open each wound. Constant opening and reopening of a wound forms a large ring of scar tissue around the wound, termed a sap pit. Generations of marmosets 'farm' the same tree, resulting in sap pits throughout the tree's height.

It is thought that marmosets adapted to eating gum because it is an important source of calcium. Calcium is required for growth and maintenance of bones and teeth. (Larger mammalian species may also eat eggshells and bones of small prey, which are even richer in calcium.)

Question 3

Think of an experiment to find out if it is likely that marmosets eat gum to obtain calcium.

Answer

You would need to find out if marmosets require more calcium than they obtain from foods other than gum. One method would be to offer marmosets two containers of drinking water, one containing plain water and the other containing calcium-enriched water, i.e. water that has additional calcium, and to monitor which one is drunk most often during a set period of time. If the calcium- enriched water is preferred, then the hypothesis that marmosets require additional calcium is supported.

In fact, this experiment has already been carried out. Researchers found that not only do marmosets preferentially drink calcium-enriched water but also lactating females show the greatest preference, suggesting that they need additional calcium for feeding their young, as indeed do most other mammals.

This experiment demonstrates that marmosets require calcium in their diet, but does it confirm the hypothesis that marmosets eat gum because it is a source of calcium? The results provide evidence that marmosets seek out sources of calcium. But for the hypothesis to be fully supported, we need to demonstrate that other foods in their diet, such as insects, are poor sources of calcium and that gum is a good source of calcium, indicating that individuals eating gum are more likely to obtain sufficient calcium than individuals that don't eat gum. Researchers measured the calcium content of different items in the diet and their measurements of insects and gum were very much along these lines. Using all this information together, we can be reasonably confident that the hypothesis is true: marmosets obtain the calcium they need by eating gum.

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